The present invention relates to silver electroplating solutions. More particularly, the present invention relates to an improved silver electroplating solution containing silver iodide, potassium iodide and a calcium salt.
The silver plating solutions most widely used are cyanide-containing baths. Although cyanide-containing silver plating baths are economical, they are poisonous and must be operated under controlled conditions. One possible alternative to the standard cyanide electroplating baths is a bath containing silver iodide. The solubility of silver iodide in water is very small, i.e., only about 0.0000028 gram per liter at 25.degree. C. However, silver iodide dissolves readily in strong solutions of potassium iodide. The increase in solubility of the silver iodide is believed to be due to the formation of argentous complexes with potassium and iodide. Silver iodide-potassium iodide solutions readily lend themselves to silver electroplating. An example of a typical silver iodide-potassium iodide electroplating bath is found in U.S. Pat. No. 1,875,664.
Various additives, such as surfactants, e.g. Tergitol NPX (available from Union Carbide Corp., Moorestown, N.J.); brighteners, e.g. ammonium thiosulfate, animal gelatin, dextrin, and naphthalene disulfonic acid; and conductivity enhancers, e.g. sodium nitrate, and potassium nitrate, can be added to the silver iodide-potassium iodide electroplating solution. Surfactants are used as wetting agents to reduce surface tension between the solution and the cathode to be plated. Brighteners, as the name suggests, add lustre to the plating. Conductivity enhancers aid in uniformity of plating. However, experiments with silver iodide-potassium iodide plating solutions, even with various additives, revealed that the electroplated silver was not as adherent, nor as ductile nor as uniform as coatings obtainable from silver cyanide baths.